Surface plasmon resonance sensors which are integrally formed are known, for example, from U.S. Pat. No. 5,912,456, which is incorporated herein by reference. A surface plasmon resonance device can be utilized for sensing because of the oscillation of a surface-plasma of free electrons which exist at a conductor-dielectric boundary and which is affected by the refractive index of material adjacent to the conductor film surface which can be detected from the other side of the surface plasmon sensor. For a given wavelength of radiation, when the angle of incidence of polarized radiation has a particular value, which is dependent upon the refractive index of the material adjacent to film, resonance occurs. Changes in the refractive index of the material causes changes in the angle at which surface plasmon resonance occurs. When polarized light strikes the thin metal film at the resonance angle, the intensity of the reflected light is minimized. The sensor works by detecting the angle at which this minimum reflections occurs, and determining therefrom the refractive index of the material adjacent to the film. This patent shows an integrally formed surface plasmon resonance sensor. The sensor shown in FIG. 2 of the patent is similar to the sensor depicted FIG. 1 of the present application except that the light in FIG. 1 is reflected off of the sample first and then off of the mirror, which is the opposite of what is shown in the patent. This reversal of elements is specifically contemplated by the patent.
This integrally formed surface plasmon sensor is shown in FIG. 1 generally as 100. The sensor comprises an integrally formed housing 102 which is made of a material which is transparent to the radiation from the light source 104. As shown, the shape of the housing 102 is generally a trapezoidal shape, although the surfaces at the top 108 and bottom 140 thereof are not necessarily parallel. Light emitted from light source 104 as 112 impinges upon a sensing surface 106 which has a surface plasmon resonance element thereon. The surface plasmon resonance element may be a thin film of copper, silver or gold having a substantially uniform thickness. The material can be applied directly to the sensing surface or can be applied to a thin glass sheet which is then attached to the sensor. The light 114 which is deflected off of the surface plasmon sensor at 106 is reflected off of a mirror 108 onto a photodetector 110. The mirror is typically a gold film on a thin glass sheet which is attached to the top of the housing 102. The photodetector 110 is typically a line sensor having N×1 pixels, where N is the number of pixels along a single horizontal line in the photodetector. As described in the patent, it is common for the light source and photodetector to be mounted on a circuit board (not shown) which is then integrally formed such as by encapsulation, within the sensor 100. Although modern manufacturing techniques can place these components on the circuit board with high accuracy, it is possible to have a tolerance of ±4 mils for a component shift, which is equivalent to ±2 pixels on the photodetector 110. In order to achieve accurate results, it is necessary to calibrate the sensor. This calibration is typically performed by placing a liquid of known refractive index, such as water, on the surface plasmon sensor 106. The light source is activated and the resulting output of the photodetector is shown as signal 120. This signal has a minimum point 122, which represents the angle at which plasmon resonance occurs. As can be seen from FIG. 1, the shape of curve 120 is the reverse of the shape of curve 124, which is the characteristic of water, due to the reflection in mirror 108. The position of a minimum point is determined and compared to the expected minimum point. As shown in FIG. 2, curves representing the response of the sensor to a liquid on the sensing surface 106 are shown generally as 200. The curve 202 is the theoretical curve for the sensor response, if the components were precisely placed. The horizontal translation along the pixel position as shown in FIG. 1 results in the horizontal translation of the curve 202 without changing the shape thereof. Thus, the curve may vary between curves 206 and 208, for example, in response to the movement of the reflection minimum point. Once the appropriate curve is determined, the response of the sensor can be calibrated by changing the parameters of the equation which define the curve 202. The need to place a liquid upon the sensing surface 106, is undesirable and a disadvantage of this technique.
Critical angle sensors are known, for example, from the U.S. Pat. No. 6,097,479, which is incorporated herein by reference. This type of sensor utilizes the measurement of the critical angle to determine the refractive index of a material. The critical angle is a function of the refractive index. Light impinging upon the material at an angle which is equal to or exceeds the critical angle will undergo total internal reflection which occurs when light rays are incident from a medium having a high index of refraction onto the medium having a lower index of refraction. The transition from transmission to total internal reflection is utilized to measure the critical angle and to calculate the refractive index therefrom. FIG. 3 of present application is similar to the FIG. 3 of the patent, with two exceptions. The first is that in the patent light impinges upon the mirror 119 before impinging upon the sample whereas in the present application, the light is shown impinging upon the sample first and then the mirror second. Secondly, the configuration of the sensor of the patent has been modified in FIG. 3 and it is similar to the configuration shown in FIG. 1.
In FIG. 3 the critical angle sensor is shown generally as 300. The sensor is enclosed in housing 302 which is generally of the same shape as the housing 102 shown in FIG. 1. It contains a light source 304 which generates light 312 which impinges upon a sample on a sensing surface 306. The surface 306 is different from the surface 106 in FIG. 1 because there is no surface plasmon resonance element thereon. A glass plate may be attached to the housing at this point, to provide a flat surface for the sample. Space between the glass plate and the housing is filled with material having the same index of refraction as the housing. The light 312 impinges upon the surface 306 will therefore enter into the sample 318 and be reflected by the difference in the index of refraction between the housing 302 and sample 318, here shown as water. The light produced by this reflection 314 impinges upon a mirror 308 at the top of the housing 302. As in FIG. 1, the mirror 308 may be a glass plate having a gold film thereon attached to the top of the housing 302. The light that impinges upon the mirror 308 is reflected as 316 and impinges upon the photodetector 110. The characteristic for water 324 will appear across the elements of photodetector 310, except that they will be reversed, because of the reflection in the mirror 308. The curve 320 shows the output of the photodetector with respect to pixel position. As can be seen, there is a minimum point 322 which is the characteristic for water and which can be utilized to calibrate the position of the characteristic curve for the sensor. Sensors of this type are of similar construction to the sensors shown in FIG. 1 and thus suffer from this same problem with respect to tolerances in the assembly of the part. The tolerances are also present in the horizontal plane and a curve fitting technique as shown in FIG. 2 can be utilized to calibrates sensors of this type as well. As with the sensor in FIG. 1, these sensors will suffer from the disadvantage of having to place a liquid upon the sensor in order for the sensor to be calibrated.